The present invention relates to membranes of fluoropolymers suitable to be used for microfiltration and ultrafiltration.
More specifically the invention relates to semipermeable porous membranes based on copolymers ethylene/chlorotrifluoroethylene (E/CTFE) which show an optimal combination of chemical, thermal and mechanical resistance. For this very good combination of properties the membranes of the invention can be used for the purification (microfiltration) of strongly aggressive chemicals such as strong acids and bases.
Membranes of fluoropolymers for the chemical purification are known in the prior art. For example porous PVDF membranes are known, prepared by casting or by plasticization/subsequent extraction of the plasticizer. The drawback of these membranes is that they cannot be used in basic environment since they show poor chemical resistance.
Membranes based on ethylene/chlorotrifluoroethylene (E/CTFE) are also known, copolymer commercially known as Halar(copyright), which are suitable for the microfiltration (purification) of strong acids and bases. See for example U.S. Pat. No. 4,623,670 and U.S. Pat. No. 4,702,836 wherein membranes for example of Halar(copyright) and the process for preparation thereof are described.
In U.S. Pat. No. 4,623,670 and U.S. Pat. No. 4,702,836 a procedure is described for preparing fluoropolymer membranes, i.e. of copolymers of ethylene with tetrafluoroethylene (E/TFE) or with chlorotrifluroethylene (E/CTFE) and homopolymers of chlorotrifluoroethylene (PCTFE), by mixing 10-60% by volume of polymer, with 7-42% by volume of inert compound (silica) and 30-75% by volume of PCTFE oligomers as plasticizer; afterwards the mixture is extruded to be plasticized at 250xc2x0 C. and pellets of plasticized compound are thus obtained useful for the subsequent obtainment of manaufactured articles such as for example compression molded plaques, hollow fibers. The fluoropolymer membrane was obtained by a first extraction of the manufactured article with 1,1,1-trichloroethane at 50xc2x0 C. for one hour to remove the plasticizer (PCTFE oligomers) and lastly by a subsequent extraction of the inert compound (silica) by NaOH at 40% at 70xc2x0 C. for one hour. In this way membranes having a porosity in the range 40-90% with a pore average size of 0.01-5xcexcm were obtained. The polymers of the aforesaid membranes are semi-crystalline compounds insoluble at room temperature and besides plasticizable at high temperature.
The drawback of the process described in the above patents is that it requires high plasticization temperatures and the use of high amounts of plasticizer formed by CTFE oligomers which is a very expensive product. If the plasticizer is used in amounts lower than 30% by volume, porous membranes are not obtained. Besides the use of high amounts of plasticizer has the drawback to require the addition of inert compound, for example silica, to give sufficient mechanical properties so to allow the extrusion of the manufactured articles to be subjected to the subsequent plasticizer extraction to obtain the porous membrane. In the extraction process a chlorinated solvent, 1,1,1-trichloroethane, is used, which, as well known, is highly toxic. The process requires also a further treatment with concentrated caustic soda for the extraction of the inert compound (silica). Therefore the process results complicated and expensive.
The need was therefore felt to have available porous membranes of fluoropolymers obtainable by a simplified and cheap process which avoided the drawbacks of the above prior art.
Membranes based on fluoropolymers ethylene/chlorotrifluoroethylene (Halar(copyright)) and their preparation process solving the above technical problem, have been unexpectedly and surprisingly found.
An object of the present invention are therefore semipermeable porous membranes having the average pore diameter in the range 0.01-3 xcexcm, more specifically 0.1-0,7 xcexcm, and a porosity in the range 30%-90%, based on semi-crystalline fluoropolymers mainly formed by ethylene copolymers with chlorotrifluoroethylene and/or tetrafluoroethylene and at least another monomer comprising:
(a) from 10 to 70%, preferably from 35 to 55%, by moles of ethylene,
(b) from 30 to 90%, preferably from 45 to 65%, by moles of a fluorinated monomer selected from tetrafluoroethylene, chlorotrifluoroethylene, or mixtures thereof,
(c) from 0.1 to 30%, preferably from 1 to 15%, by moles, with respect to the total amount of monomers (a) and (b), of a hydrogenated monomer of formula:
CH2xe2x95x90CHxe2x80x94(CH2)nR1xe2x80x83xe2x80x83(I) 
wherein R1=xe2x80x94OR2, or xe2x80x94(O)tCO(O)pR2 wherein t and p are integers equal to 0.1 and R2 is a linear or branched C1-C20 alkyl, or a cycloalkyl, optionally containing heteroatoms and/or chlorine atoms, preferably the heteroatoms are O or N;
R2 can optionally contian one or more functional groups, preferably selected from OH, COOH, epoxy, ester and ether group; R2 can optionally contain double bonds, or it can be H;
n is an integer in the range 0-10.
More preferred compositions of the invention copolymers are the following:
(a) from 35 to 45%, by moles of ethylene,
(b) from 55 to 65% by moles of a fluorinated monomer selected from tetrafluoroethylene, chlorotrifluoroethylene, or mixtures thereof,
(c) from 3.5 to 11.5% by moles, with respect to the total amount of monomers (a) and (b), of a hydrogenated monomer of formula (I).
The preferred comonomers (c) are for example selected from the following classes:
1) Acrylic monomers having general formula:
CH2xe2x95x90CHxe2x80x94COxe2x80x94Oxe2x80x94R2 
wherein R2 has the above meaning.
Ethylacrylate, n-butylacrylate, acrylic acid, hydro-xyethylacrylate, hydroxypropylacrylate, (hydroxy)ethyl-hexylacrylate can for example be mentioned.
2) Vinylether monomers having general formula:
CH2xe2x95x90CHxe2x80x94Oxe2x80x94R2 
wherein R2 has the above meaning.
Propylvinylether, cyclohexylvinylether, vinyl-4-hydroxybutylether can for example be mentioned.
3) Vinyl esters of the carboxylic acid having general formula:
CH2xe2x95x90CHxe2x80x94Oxe2x80x94COxe2x80x94R2 
wherein R2 has the above meaning.
Vinyl-acetate, vinylpropionate, vinyl-2-ethylhexannoate can for example be mentioned.
4) Unsaturated carboxylic acids having general formula:
CH2xe2x95x90CHxe2x80x94(CH2)nxe2x80x94COOH 
wherein n has the above meaning.
For example vinylacetic acid.
Preferably as comonomers the acrylic ones of class 1) are used.
The membranes of the present invention are obtained from the above mentioned fluoropolymers which are semi-crystalline and therefore show a melting point determinable by calorimetric analaysis (DSC).
The membranes of the present invention are obtained starting from the above semi-crystalline fluoropolymers by their plasticization, with plasticizers suitable to obtain a solution, subsequently formed in a membrane and then subjected to the plasticizer extraction. In particular the process comprises the hot mixing of the aforesaid polymers with one or more plasticizers. The temperature at which the solution is prepared ranges from 140xc2x0 C. to 195xc2x0 C., preferably from 160xc2x0 C. to 180xc2x0 C.
The so obtained solution is worked by extrusion, molding by injection or compression to give the desired shape to the membrane. Then the so obtained membrane is dipped into a solvent to extract the plasticizer. It is possible to extract at room temperature obtaining a complete plasticizer extraction in a time ranging from some minutes to some hours, depending on the thickness, the type of the extractant and stirring. Generally times of few minutes are sufficient to completely extract the plasticizer. After extraction the porous invention membrane is obtained.
The plasticizers used in the process are selected from the plasticizers known in the prior art having a vapour pressure lower than 5 mm Hg, preferably lower than 2 mm Hg, at the temperature of 160xc2x0 C.
The plasticizer amount ranges between 10% and 70% by weight with respect to the fluoropolymer, preferably between 25% and 65%, more preferably between 35% and 55%, depending on the porosity of the final membrane one wants to obtain.
Hydrogenated plasticizers are preferably used. Citrates, phthalates, trimellitates, adipates can for example be mentioned. Preferably citrates and trimellitates are used, in particular acetyltri-n-butylcitrate and trihexyltrimellitate. The hydrogenated plasticizers preferably used in the invention are available on the market at low costs. This represents an advantage of the present invention.
As extraction solvents, solvents wherein the plasticizer is soluble, but which are not compatible with the fluoropolymer, so as not to cause the swelling thereof, are used.
The most commonly used class of solvents is that of aliphatic alcohols, preferably having a short chain, for example from 1 to 6 carbon atoms, more preferably methanol and isopropanol.
The mebranes of the present invention have a high chemical resistance. Dipping tests in an aqueous solution of sodium hydroxide (NaOH) at 30% by weight at the temperature of 50xc2x0 C. have shown that even after one month the membrane substantially shows the same initial mechanical properties, for example tensile properties, such as modulus, stress and strain yield, stress and elongation at break. Besides the membrane does not show phenomena of colour variation (discoloration).
Some Examples follow for illustrative and not limitative purposes of the invention.